Hello and thank you for allowing me to address your legal question.
How specific was the agreement? Was it truly as broad and general as you stated here, or was it more specific? For example, was the term "successful" defined? Generally speaking, the broader the language, the less likely it will be enforced in court because it will be too ambiguous. If you can prove specific terms, however, then the agreement may be enforced. So, if you take it to court be prepared to prove details such as length of time (if applicable), dollar figures, position, etc.
An audio recording can be used to prove the existence of a contract, so long as the audio itself can be authenticated. Authenticating a recording is as simple as somebody familiar with your employer’s voice stating “yes, that voice is the defendant’s voice.”
DISCLAIMER: Please understand that the complexities of most legal problems cannot be sufficiently addressed in this setting. Accordingly, my post is intended as general information only, and should neither be construed as specific legal advice, nor as an adequate substitute for the retention of legal counsel.